It has been known that a partially saponified vinyl alcohol polymer is used as a dispersion stabilizer for suspension polymerization for a vinyl compound (for example, vinyl chloride).
A partially saponified PVA has, however, low solubility in water, so that handleability is unsatisfactory. Therefore, in order to improve handleability, attempts have been made for improving solubility of a PVA in water, including introduction of a hydrophilic polyoxyalkylene group to a side chain of the PVA or introduction of an ionic group.
For example, there have been proposed the use of a PVA which is of a low polymerization degree and a low saponification degree and contains oxyalkylene groups in side chains (see Patent Reference No. 1), or a PVA having ionic groups (see Patent Reference Nos. 2 and 3).
However, the dispersion stabilizers described in Patent Reference Nos. 1 to 3 are not necessarily satisfactory in terms of required performance; specifically, (1) even with a small amount, polymer particles obtained are highly absorbing a plasticizer and easily processable, (2) remaining monomer components can be easily removed, (3) polymerization stability is so high that a less amount of coarse particles are formed, and so on. In particular, the requirement for removal of monomer components in (2) has been very strict under stricter regulations for the amount of residual monomers in a resin product and so on. Thus, requirements for energy reduction and time reduction are very strong, and a dispersion stabilizer which allows for easy removal of residual monomers from a vinyl chloride polymer is needed. At present, it cannot be said that a dispersion stabilizer successfully satisfying the requirements is provided, including the dispersion stabilizers of Patent Reference Nos. 1 to 3.